(1) Field of the Invention
The present invention relates generally to composite castings and solders including brazing, and more particularly the present invention relates to composite castings and solders which contain organofunctional inorganic oxide particles. The invention further relates to a lead-free composite solder.
(2) Description of the Related Art
Light weight castings are increasingly used as substitutes for cast iron for reducing weight. Castings have many uses, especially in the automotive and aerospace industries. Often the light weight castings do not have the performance or reliability of the iron castings. One approach to this problem is to provide high strength inserts at critical stress points in the light weight castings.
U.S. Pat. Nos. 6,484,790 and 6,443,211 to Myers, et al. teach a method for forming light-weight composite metal castings incorporating metallurgically bonded inserts for a variety of applications. A casting method includes the step of coating the insert with a first layer under conditions including sufficient temperature to cause a portion of the layer to be sacrificed by dissolving into the cast metal material while leaving at least a portion of the first layer as a diffusion barrier between the insert and the cast material.
U.S. Pat. No. 6,006,819 to Shimizu, et al. teaches an aluminum-based composite member having an increased strength of bond between an aluminum-based body and a cast iron material portion which is incorporated into the aluminum-based body by casting.
U.S. Pat. No. 5,333,668 to Jorstad, et al. teaches a process for coating a ferrous or aluminum article, such as an engine cylinder liner insert, to provide a metallurgical bond with aluminum alloy material cast around the article.
Lead-free electronic solders are low melting alloy systems. However, their service performance in modern microelectronic applications requires that they have better thermomechanical (TMF) capabilities, good dimensional stability, and reduced electromigration. Good TMF performance is important for automotive, aerospace/defense, and consumer electronic applications, since these situations involve severe thermal excursions. In typical microelectromechanical systems (MEMS), the dimensional stability of the interconnects becomes important, since the lines in printed circuit boards that carry electrical current are placed very close to each other. High current densities encountered in such MEMS, and current generating applications, warrant solutions for electromigration problems.
Composite solders are solders with intentionally incorporated reinforcements. There have been several attempts to incorporate compatible reinforcements in solder systems. Such methods include incorporation of intermetallic (IMC) reinforcements by in-situ or mechanical mixing methods. Mechanical mixing methods can incorporate IMC directly to the solder paste, or convert the metallic powder added to the paste to IMC by reacting with the molten solder during the reflow process. However, such reinforcements tend to be several microns in size. In addition, IMC reinforcements tend to coarsen during service and affect their effectiveness. Service performance improvements achieved by IMC reinforcements tend to be varied depending on the type of the IMC reinforcement, method used to incorporate such reinforcements, and their coarsening kinetics during service. As a consequence, IMC particulate reinforced composite solders have not been implemented in the actual electronic solder interconnects. With the rapid advance in the miniaturization of the electronic components a need for solders with sub-micron reinforcements has developed.
An alternate approach is to incorporate inert particulate reinforcements in the solder matrix. One such attempt is to incorporate iron particulate in the presence of a magnetic field (M. McCormack, S. Jin, and G. W. Kammlott, “Enhanced Solder Alloy Performance by Magnetic Dispersions”, IEEE Transactions on Components, Packaging, and Manufacturing Technology—Part A, 17 (3), pp. 452-457, 1994.) Incorporation of ceramic particulates such as alumina in the electronic solders has also been attempted. One problem with this methodology is the agglomeration of the reinforcements during the reflow process making them to acquire larger sizes with pores. Attempts such as mechanical working by rolling to break up and disperse these agglomerated particulates have been tried with associated problems of interface cracking between the reinforcements and the matrix (H. Mavoori and S. Jin, “New Creep-Resistant, Low Melting Point Solders with Ultrafine Oxide Dispersions”, J. Electr. Mater., 27(11): pp.1216-1222, 1998). Another serious problems associated with incorporation of such inert reinforcements is lack of any chemical bonding between the reinforcement and the solder matrix, which makes them not very effective to enhance the service performance (H. Mavoori and S. Jin). As a consequence such solders with inert reinforcements have not been implemented in practice.
Another approach that is vigorously pursued is alloying Sn—Ag based solders with small quantities Cu, Ni, rare-earth elements etc. (C. M. Miller, I. E. Anderson, and J. F. Smith, “A Viable Tin-Lead Solder Substitute: Sn—Ag—Cu”, J. Electr. Mater., 23(7), pp.595-601, 1994., F. Guo, S. Choi, T. R. Bieler, J. P. Lucas, A. Achari, M Paruchuri and K. N. Subramanian, “Evaluation of Creep Behavior of Near Eutectic Sn—Ag Solders Containing Small Amount of Alloy Additions”, Materials Science and Engineering, A351, pp.190-199, 2003, J. G. Lee, F. Guo, S. Choi, K. N. Subramanian, T. R. Bieler, and J. P. Lucas, “Residual Mechanical Behavior of Thermomechanically Fatigued Sn—Ag Based Solder Joints”, J. Electr. Mater., 31(9), pp.946-952, 2002., C. M. L. Wu, C. M. T. Law, D. Q. Yu, and L. Wang, “The Wettability and Microstructure of Sn—Zn-RE Alloys”, J. Electr. Mater., 32(2): pp.63-69, 2003., C. M. L. Wu, D. Q. Yu, C. M. T. Law, and L. Wang, “Improvements of Microstructure, Wettability, Tensile and Creep Strength of Eutectic Sn—Ag Alloy by Doping with Rare-Earth Elements”, Journal of Materials Research, 17(12), pp.3146-3154, 2002). Although these ternary and quaternary alloy solders can produce binary and ternary IMC precipitates, it is difficult to control their size and distribution during the reflow process. Such alloying also alters the melting temperature affecting the reflow parameters warranting changes in processing parameters and methodologies.
With the rapid advance in the miniaturization of the electronic components a need for solders with sub-micron reinforcements becomes a necessity. Such sub-micron size reinforcements when present at the grain boundaries can minimize grain boundary sliding, the predominant mode of TMF damage, during the high temperature dwell in a TMF cycle, by keying the grain boundaries. Such an approach is employed in nickel and cobalt based super alloys used in high temperature service environments. These reinforcements can minimize the TMF damage and improve their service reliability, and also improve the dimensional stability that is essential for MEMs and microelectronics applications.
In microelectronic applications, as the current density increases the migration of ions between electrodes cause voiding that results in the failure of the solder joint. Similar issues in computer industry have been successfully tackled by incorporation of copper atoms in the grain boundaries of aluminum lines. However, there is no known solution for the same problem in the case lead-free solders. The sub-micron sized reinforcements that can result from the proposed method of reinforcing the solders can provide a solution to the electro migration problem.
U.S. Pat. No. 5,127,969 to Sekhar teaches a solder composition with a continuous phase and a disperse phase. The disperse phase is a reinforcing material in particulate or fibrous form comprising graphite, silicon carbide, a metal oxide, an elemental metal, and/or a metal alloy. The reinforcing material remains in particulate or fibrous form as a disperse phase. While Sekhar teaches particulate and fibrous reinforcements ranging from submicron to sixty microns, it does not teach of nano-sized particle reinforcements.
U.S. Pat. Nos. 5,928,404 and 6,360,939 to Paruchuri, et al. teach an electrical solder paste having primary solder powder and an additive metal powder component which does not melt during the soldering process due to it having a melting point substantially higher than the melting point of the primary powder.
U.S. Pat. No. 6,340,113 to Avery, et al. teaches solder compositions which are composed of particles of a first metal coated with a second metal, or a salt solution or suspension of a second metal. The metals are chosen such that their individual melting points are higher than the melting points of the alloys formed when they are combined. The coated particles are heated and melting occurs at the interfaces between the core materials and their coatings. The particles fuse together into a porous metal foam.
U.S. Pat. No. 6,521,176 to Kitajima, et al. teaches a lead-free solder alloy with respective concentrations set such that the lead-free solder alloy has a melting temperature lower than a predetermined heat-resistant temperature of a work to be soldered.
U.S. Pat. No. 5,866,044 to Saraf, et al. teaches an electrically conductive paste which includes a thermoplastic polymer, a conductive metal powder and an organic solvent system. The thermoplastic polymer is selected from the group consisting of a poly(imide urea), a poly(ether siloxane), a poly(styrene butadiene), a poly(styrene isoprene), a poly(acrylonitrile butadiene), a poly(ethylene vinyl acetate) and a polyurethane.
While the related art describes alternative ways to address improving service performances of solders, there is still a need for a superior solution to this long unsolved problem. Therefore, it is desirable to develop composite solder compositions with sub-micron reinforcements which have improved service performance. It is further desirable that composite solder compositions be free of lead. Environmental lead release is a growing concern considering the inherent toxicity of lead and the rapidly expanding utilization of electronic circuitry in all aspects of modern life. Solder compositions which reduce the amount of lead while retaining properties such as melting temperature, solderability, fatigue behavior, and processing parameters of lead solders would address a long unsolved problem of toxic waste disposal.